A man shot by police in Ferguson, Missouri who said he had opened fire on them is in a "critical, unstable" condition and due to have surgery.
He was at a protest to mark the first anniversary of the killing of unarmed black teenager Michael Brown.
Police said he was armed with a stolen gun and they had been tracking him. Four officers have been placed on administrative leave.
Photos from the scene showed a young black man lying bloodied on the ground.
He was face down and handcuffed. In a video posted on Twitter, a man can be heard urging the police to "please, get him some help", as the injured man lies still.
The wounded man's name and age have not been released by police. The St Louis Post-Dispatch identified him as 18-year-old Tyrone Harris, having spoken to his father, the paper said.
The shooting of 18-year-old Michael Brown by white police officer Darren Wilson in August 2014 sparked demonstrations across America.
It fuelled a national protest movement against racial bias by police.
'This is no longer a peaceful protest'
Shots were heard at around 11.15pm on Sunday (local time) as a large crowd gathered on West Florissant Avenue, at the end of a sombre, peaceful day of commemorations.
Mr Belmar told reporters that after "an exchange of gunfire between two groups" the suspect, believed by police to be armed, left and encountered officers in an unmarked SUV with flashing lights.
The suspect fired on the vehicle, Mr Belmar said, and the plainclothes detectives returned fire from inside the van.
The man ran behind a building and fired again at the officers, the police chief said, and all four fired back, wounding him.
The officers were not wearing body cameras, he added.
Mr Belmar described the incident was "a tragedy for the family of this man, and the officers involved."
There was a "small group of people out there intent on making sure we don't have peace that prevails," he said.
In the early hours of the morning, police used loudspeakers to urge people to disperse, saying "this is no longer a peaceful protest" and "we intend to use chemical munitions".
Eyewitnesses on Twitter reported police using tear gas to disperse protesters.
Earlier on Sunday, hundreds of people stood silent for four and a half minutes at the spot where Michael Brown was killed, representing the number of hours that his body lay in the street unattended.